This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-225812, filed Jul. 26, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a cathode ray tube apparatus, and more particularly to a cathode ray tube apparatus provided with an electron gun structure having an electric field expansion type main lens with a large diameter.
2. Description of the Related Art
In recent years, demands for realizing the high resolution of a cathode ray tube apparatus has been increased, and a beam spot diameter on a phosphor screen which is a large factor for determining the resolution is determined by the focusing performance of an electron gun structure for emitting electron beams.
This focusing performance is generally determined by a diameter of a main lens, a virtual object image diameter relative to the main lens, the magnification of the main lens and others. That is, the beam spot diameter can be decreased as the diameter of the main lens is increased, as the virtual object image diameter is reduced and as the magnification of the main lens is reduced, thereby improving the resolution.
For example, as shown in FIGS. 5 and 6, there is known an electron gun structure having an electric field expansion type main lens with a large diameter. This electron gun structure includes two intermediate electrodes Gm1 and Gm2 arranged between a focus electrode G5 and an anode electrode G6. An electric field between the focus electrode G5 to the anode electrode G6 is expanded in a traveling direction of electron beams by applying a potential between the focus electrode G5 and the anode electrode G6 to the intermediate electrodes Gm1 and Gm2.
As described above, the main lens in this electron gun structure expands an electric field in the main lens along the electron beam traveling direction and forms the gentle potential gradient, thereby constituting a long-focusing lens. As a result, the beam spot diameter on the phosphor screen is decreased, and the resolution is improved.
Jpn. Pat. Appln. KOKAI Publication No. 64-38947 discloses an electron gun structure including two intermediate electrodes. In this electron gun structure, an application voltage of the focus electrode is approximately 6 kV to 9 kV, and an application voltage of the anode electrode is approximately 25 kV to 30 kV.
A voltage which is approximately 40% of the anode voltage is applied to a first intermediate electrode arranged on the focus electrode side, and a voltage which is approximately 65% of the anode voltage is applied to a second intermediate electrode arranged on the anode electrode side. These two intermediate electrodes are constituted in such a manner that their electrode lengths in the electron beam traveling direction are equal to each other.
However, in order to sufficiently exploit the characteristic of such an electric field expansion type main lens, the electrode length of each electrode, an opening diameter and the potential distribution thereof must be appropriately set. In the above-described structure, however, the density of the potential gradient differs in the vicinity of the first intermediate electrode and the second intermediate electrode.
That is, a difference in potential between the electrodes on the both sides of the first intermediate electrode, namely, a difference between voltages applied to the focus electrode and the second intermediate electrode (assuming that the focus voltage is 25% of the anode voltage, a difference in potential is 65%xe2x88x9225%=40%) dominantly acts on the electric field in the vicinity of the first intermediate electrode. Further, a difference in potential between the electrodes on the both sides of the second intermediate electrode, namely, a difference between voltages applied to the anode electrode and the first intermediate electrode (a difference in potential is 100%xe2x88x9240%=60%) dominantly acts on the electric field in the vicinity of the second intermediate electrode. Thus, when the electrode length of each of the first intermediate electrode and the second intermediate electrode and a distance between these electrodes are equal to each other, the density of the potential gradient in the vicinity of the second intermediate electrode becomes stronger than that of the potential gradient in the vicinity of the first intermediate electrode. Therefore, the potential gradient constituting the electric field expansion lens becomes locally uneven.
In order to make the electric field expansion type lens to function as a lens with a larger diameter (long-focusing lens), the electric field expansion type lens must be constituted as if it is a part of the central axis of a large lens. That is, when the potential gradient in the electric field expansion type lens is uniform, the lens with a larger diameter can be obtained, and the aberration component accepted by the electron beams can be further reduced.
Therefore, the electric field expansion type lens having the considerable unevenness in the potential gradient disclosed in the prior art described above can be said that its lens is not constituted as a lens with a sufficiently large diameter.
Furthermore, in the above-mentioned electron gun structure, an opening diameter and an electrode length of each intermediate electrode are not explicitly described. However, the appropriate relationship is required for the opening diameter and the electrode length of each of these intermediate electrodes.
For example, if the electrode length is sufficiently long with respect to the opening diameter of the intermediate electrode, the following problem occurs. That is, as shown in FIG. 4, when the electrode length L1 is long with respect to the opening diameter "PHgr" to some extent, the discontinuity is produced in the potential gradient in the vicinity of the center of that intermediate electrode. Therefore, the gentle potential gradient formed from the focus electrode to the anode electrode is interrupted in the vicinity of the center of that intermediate electrode. The electric field expansion type lens having such discontinuity can be also said that its lens is not constituted as a lens with a sufficiently large diameter.
As described above, in the prior art electron gun structure, since the opening diameter and the electrode length of each electrode constituting the electric field expansion type lens and a distance between the respective electrodes are not appropriately set, the potential gradient constituting the electric field expansion type lens becomes uneven or the potential gradient is interrupted. Therefore, there occurs a problem that the lens with a sufficiently large diameter can not be constituted.
In view of the above-described problems, it is an object of the present invention to provide a cathode ray tube apparatus which can fully bring out a lens characteristic of the electric field expansion type main lens and obtain an excellent image characteristic in the entire phosphor screen.
In order to solve the above-described problem and achieve this aim, there is provided a cathode ray tube apparatus according to claim 1, comprising:
an electron gun structure having an electron beam formation portion for generating at least one electron beam and a main lens portion for focusing an electron beam generated from the electron beam formation portion onto a phosphor screen; and a deflecting yoke for generating a deflecting magnetic field for deflecting an electron beam emitted from the electron gun structure in the horizontal direction and the vertical direction,
wherein the main lens portion is constituted by including: at least one focus electrode to which a focus voltage on a first level is applied; at least one anode electrode to which an anode voltage on a second level higher than the first level is applied; and at least two auxiliary electrodes to which a voltage whose level is higher than the first level and lower than the second level is applied, and
wherein an electrode length of each of at least two auxiliary electrodes along a traveling direction of the electron beam differs in accordance with a difference in potential between electrodes arranged at front and rear positions in an electron beam traveling direction of each electrode.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.